1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor light emitting device.
2. Description of the Related Art
A group III nitride compound semiconductor light emitting device has such a structure that an active layer is interposed between an n-type semiconductor layer and a p-type semiconductor layer. In order to increase a light emission efficiency in the light emitting device, the active layer is set to have a multiple quantum well structure in some cases. Also in the n-type semiconductor layer and the p-type semiconductor layer, a superlattice structure is employed in some cases. By employing the superlattice structure in the n-type semiconductor layer and the p-type semiconductor layer, the resistance of each layer is reduced so that an electron injection efficiency and a hole injection efficiency to the active layer can be enhanced, respectively.
As documents related to the invention, see JP-A-10-326943, JP-A-11-340509, JP-A-2002-198314, JP-A-2002-319743 and JP-A-2003-17746.
The inventors have vigorously made investigations in order to enhance the light emission efficiency of the light emitting device. According to the investigations of the inventors, also in the case in which the p-type semiconductor layer has a superlattice structure in which an AlGaN layer and an InGaN layer are laminated repetitively, for example, the hole injection efficiency cannot be enhanced considerably. As a cause, it can be supposed that a p-type impurity such as Mg with which the p-type semiconductor layer is doped is diffused into the active layer. When the p-type impurity is diffused into the active layer, the quality of the crystal of the active layer is deteriorated, causing a reduction in the light emission efficiency.
Moreover, the active layer generally has a compound semiconductor composition which does not contain Al such as a GaN layer or an InGaN layer. On the other hand, a layer containing Al is used in the p-type semiconductor layer containing the superlattice structure. For this reason, there is a possibility that lattice mismatching might be caused with a change in the composition.